1. Field of the Invention
The present invention relates to a powdery particle conveying system which conveys powdery particles such as flour or livestock feed to a reservoir tank and a roots blower operating method for use in the system.
2. Description of the Prior Art
An explanation will be made by way of conveyance of flour in a flour mill. In the flour mill, numerous kinds of flour are milled, and then they are reserved in different tanks according to the kinds of flour. The conveyance to a reservoir tank is carried out by using a roots blower. In the roots blower, a fan is rotated by a motor, so as to generate a compressed air current, which is press-fed into a conveying pipe, and thus the flour in the air current is conveyed to a predetermined reservoir tank.
One roots blower or a plurality of roots blowers are installed in the flour mill. The conveying pipe extends from the roots blower to the reservoir tank. Switch valves are disposed at branch points from the conveying pipe to the reservoir tanks. A conveying channel to each of the reservoir tanks is defined by switching the switch valves, and consequently, the flour is conveyed to a desired reservoir tank.
Specifically, as illustrated in FIG. 1, a rotary valve 42 having an air sealing function is disposed under a hopper 40 for holding the milled flour therein, and further is connected to a conveying pipe 46. A measuring unit 41 is disposed downstream of the hopper 40. In the meantime, there is provided a roots blower 44. Powdery particles are press-fed from the hopper 40 through the conveying pipe 46 by a compressed air current generated by the roots blower 44. A plurality of reservoir tanks 30 are installed at positions apart from the hopper 40. Switch valves 48 are disposed at branch points from the conveying pipe 46 to the reservoir tanks 30. The flour is conveyed to each of the reservoir tanks 30 by operating the switch valves 48.
The roots blower 44 is required to have a sufficient capacity to convey the flour to any of the reservoir tanks 30. Therefore, the roots blower 44 is designed to have such a capacity or volume as to convey the flour to a reservoir tank 30B located at a remotest or highest position among the reservoir tanks 30, that is, a reservoir tank requiring the largest conveying load.
In the above-described prior art powdery particle conveying system, the roots blower having such a capacity as to sufficiently convey the powdery particles even to the reservoir tank 30B located at the remotest position is operated with its rated output, and thus the powdery particles are conveyed to all of the reservoir tanks. That is to say, the powdery particle conveying system is operated with the roots blower driving motor at the same frequency of a power source even in the case where the flour is conveyed to a reservoir tank 30A located at a position nearest the roots blower 44 as a frequency of the power source at which the flour is conveyed to the reservoir tank 30B, and therefore energy is wastefully used. In this case, the conveying load. In contrast, the rotating output of the motor is reduced so as to convey the powdery particles while reducing an electric power consumption in the case of the small conveying load; thus reducing energy consumption.
Other methods for changing the output of the roots blower include a method in which a conveying state is observed while reducing the speed of the roots blower driving motor according to the conveying load, so as to change the speed to a lowest speed at which the conveyance can be maintained; and a method in which the output of the roots blower required for each of the conveying loads is previously calculated, and then it is stored in a memory device or the like, so that the output of the roots blower with respect to a conveying load according to powdery particles to be conveyed or conveying conditions is read out of the memory device or the like when the powdery particles to be conveyed or the conveying conditions are input, and thus the output of the roots blower is automatically changed.
A similar problem arises not only because of the difference in conveying distance but also the conveying quantity or kind of powdery particles to be conveyed (for example, flour, wheat bran, flour grains and the like) or the magnitude of a conveying load such as a pressure loss on a conveying channel (for example, the bend of the conveying channel, the number of switch valves, the property of the inner surface of the conveying pipe, i.e., a surface roughness, and the like).